In recent years, a flexible device in which a functional element such as a semiconductor element, a display element, or a light-emitting element is provided over a substrate having flexibility (hereinafter also referred to as a flexible substrate) has been developed. Typical examples of the flexible device include a lighting device, an image display device, a variety of semiconductor circuits including a semiconductor element such as a transistor, and the like.
As a method for manufacturing a device using a flexible substrate, a technique has been developed in which a functional element such as a thin film transistor or an organic EL element is formed over a substrate such as a glass substrate or a quartz substrate, and then the functional element is transferred to a flexible substrate. This method needs a separation step of separating, from the substrate, a layer to be separated that includes the functional element.
For example, in a separation technique that is disclosed in Patent Document 1 and uses laser ablation, first, a separation layer including a layer containing tungsten, a layer containing tungsten oxide, and the like is provided over a substrate, a layer to be separated that includes a thin film element is provided over the separation layer, and the layer to be separated is made to adhere to a transfer body with the use of an adhesive layer. Then, the separation layer is ablated by laser light irradiation, so that separation is made to occur in the separation layer.
As a method for manufacturing a light-emitting device using a substrate having flexibility, a technique in which a separation layer is formed over a substrate, e.g., a glass substrate or a quartz substrate, a semiconductor element such as a thin film transistor is formed over the separation layer, and then, the semiconductor element is transferred to another substrate (e.g., a substrate having flexibility) has been developed (see Patent Document 1).